This invention relates generally to rechargeable electrochemical cells and more particularly to rechargeable electrochemical cells in which an ionic-conducting polymer-based solid gel membrane is used as a separator.
Electrochemical devices generally incorporate an electrolyte source to provide the anions or cations necessary to produce an electrochemical reaction. A zinc/air system, for example, requires the diffusion of hydroxide anions, and typically will incorporate an aqueous potassium hydroxide solution as the electrolyte. The lifetime of this battery is however, limited for several reasons. First, the naked zinc anode is corroded by both the aqueous electrolyte and air. Second, the air channels of the air cathode gradually become blocked by water from the electrolyte solution and third, the electrolyte solution becomes contaminated with zinc oxidation product that diffuses from the anode.
Various methods have been used to address the many problems associated with the use of aqueous electrolytes in zinc anode based systems such as zinc/air fuel cells. Additives, for example, have been introduced into the electrolyte solution to extend its lifetime and to protect the anode from corrosion. U.S. Pat. No. 4,118,551 discloses the use of inorganic additives such as mercury, indium, tin, lead, lead compounds, cadmium or thallium oxide to reduce corrosion of a zinc electrode. Many of these additives however, are expensive and more significantly, are very toxic. U.S. Pat. No. 4,378,414 discloses the use of a multi-layer separator between the positive and negative electrodes to reduce corrosion of the anode and contamination of the electrolyte by zinc oxidation products. In addition, hydrophobic materials have been introduced into zinc/air devices to prevent water permeation into the air channels of the cathode. Introduction of hydrophobic materials is however, a difficult process and may result in decreased performance of the cathode.
In addition to zinc/air systems, other metal/air systems, such as aluminum/air, lithium/air, cadmium/air, magnesium/air, and iron/air systems, also have the potential for many different applications due to their theoretically high ampere-hour capacity, voltage, and specific energy. In actual practice however, these very promising theoretical values are greatly reduced due to the corrosion of the metal anode in the electrolyte.
A solid state hydroxide conductive electrolyte polybenzimidazole (xe2x80x9cPBIxe2x80x9d) film is disclosed in U.S. Pat. No. 5,688,613 and comprises a polymeric support structure having an electrolyte active species dispersed therein, wherein the polymer structure is in intimate contact with both the anode and the cathode. This PBI film, however, does not absorb water and therefore, does not hold water within the membrane, causing it to dry out quickly.
U.S. Pat. No. 3,871,918 discloses an electrochemical cell embodying an electrode of zinc powder granules suspended in a gel comprised of methylenebisacrylamide, acrylic acid and acrylamide. Potassium hydroxide serves as the electrolyte, and is contained within the gel.
With regard to devices that rely on the conduction of cations, while there has been a significant amount of research in this area, most proton conducting membranes are very expensive to produce and typically do not function at room temperature. In the 1970""s for example, a fully fluorinated polymer membrane, NAFION(copyright) (DuPont, Wilmington, Del. USA) was introduced and has served as the basis from which subsequent proton conducting membranes have evolved.
U.S. Pat. No. 5,468,574 discloses a proton conductive membrane that is characterized as a highly sulfonated polymeric membrane composed of block copolymers of sulfonated polystyrene, ethylene and butylene blocks. In 1997, NASA""s Jet Propulsion Laboratory disclosed the development of an improved proton conductive membrane composed of sulfonated poly(ether ether ketone), commonly known as H-SPEEK.
The separator in a cell or battery physically separates and electrically insulates electrodes of different polarity. While serving as a barrier to the transport of active materials of the different electrodes, a separator should also provide ionic conduction. Good ionic conductivity is necessary to ensure that an electrochemical cell/battery is capable of delivering usable amounts of power for a given application.
In a rechargeable electrochemical cell, a separator is also used to prevent short circuiting caused by metal dendrite penetration during recharging. For example, in rechargeable zinc/air cells, zinc on the surface of the negative zinc electrode (anode) is dissolved as zincate ion into the electrolyte solution during discharge. Then, during the charge, when the charging current is typically below 20 mA/cm2, depending on the particular anode used, the zincate ion forms dendritic zinc, which is needle-like and grows from the negative electrode toward the charging electrode. Unfortunately, these needle-like structures can pierce through conventional separators causing an internal short circuit. The service life of the cell is consequently terminated. In addition to preventing dendrite penetration, the separator must allow for the exchange of electrolytic ions during both discharging and charging of the cell.
The most commonly used separators in rechargeable cells are porous insulator films of polyolefins, polyvinyl alcohol (PVA), nylon, or cellophane. Acrylic compounds may also be radiation-grafted onto these separators to make them more wettable and permeable to the electrolyte. Although much work has been done to improve the performance of separators, dendrite penetration problems are frequently encountered with these and other conventional separators, as well as problems involving diffusion of reaction products such as the metal oxide to remaining parts of the cell.
With conventional separators, controlling the pore size of the separator is the only effective way to avoid dendrite penetration and prevent product diffusion. By doing this, however, the ionic conductivity of the separator is also greatly reduced. This creates a bottleneck for high charging-discharging current density operations, important considerations for use in some applications, such as in electrical vehicles.
U.S. Pat. No. 5,549,988 discloses an electrolyte system separator disposed between the cathode and anode of a rechargeable electrochemical battery. The electrolyte system includes a polymer matrix prepared from polyacrylic acid or derivatives thereof. An electrolyte species, such as KOH or H2SO4, is then added to the polymer matrix to complete the system. However, as reported in the patent, the measured ionic conductivities of the disclosed electrolyte-polymer films are low, ranging from 0.012 S/cm to 0.066 S/cm. Although these conductivities are acceptable for some applications, they are inadequate for other high rate operations including electrical vehicles.
An electrochemical reaction is also involved in the function of electrochromic devices (ECD""s). Electrochromism is broadly defined as a reversible optical absorption change induced in a material by an electrochemical redox process. Typically, an electrochromic device contains two different electrochromic materials (ECM""s) having complementary properties; the first is generally reduced, undergoing a color (1)-to-color (2) transition during reduction, while the second material is oxidized, undergoing a similar transition upon the loss of electrons.
Basically, there are two types of electrochromic devices, depending upon the location of the electrochromic materials within the device. In a thin-film type device, the two ECM""s are coated onto the two electrodes and remain there during the redox coloration process. In a solution-phase device, both ECM""s are dissolved in an electrolyte solution and remain their during the coloration cycle. The solution-phase device is typically more reliable and has a longer lifetime, however, in order to maintain the colored state, an external power source must be continuously applied. As the thin-film type device does not need an external power source to maintain its colored state, power consumption is greatly reduced, making this an advantage for such energy-saving applications as smart windows. The drawback of the thin-film type device is that it has a short lifetime. After a certain number of cycles, ECM films can lose contact with the electrode, or they may no longer be capable of phase change and the device expires.
With regard to solution-phase devices, U.S. Pat. No. 5,128,799, for example, discloses a method of reducing the current required to maintain the colored state which involves the addition of gel into the device. While reducing energy consumption however, the addition of the gel into the device also greatly reduces the switching speed of the device. With regard to thin-film devices, attempts to extend the lifetime of the device have included changes to the crystal structure of the film. While such changes have increased the lifetime of thin-film devices to an extent, the typical lifetime of such devices is still not satisfactory.
The foregoing problems thus present major obstacles to the successful development and commercialization of fuel cell technology, a green energy source, and of electrochromic devices such as smart windows and flat panel displays, which have several energy-saving, decorative, and information display applications. With respect to the problems associated with rechargeable electrochemical cells, it is clear that there is a great need for a separator that can provide improved ionic conductivity while providing an effective barrier against the penetration of metal dendrites and the diffusion of reaction products.
The present invention provides polymer-based solid gel membranes that contain ionic species within the gel""s solution phase and that are highly conductive to anions or cations. In accordance with the principles of the invention, solid gel membranes may be produced for use in such power sources as, for example, metal/air (e.g. zinc/air, cadmium/air, lithium/air, magnesium/air, iron/air, and aluminum/air), Zn/Ni, Zn/MnO2, Zn/AgO, Fe/Ni, lead-acid, Ni/Cd, and hydrogen fuel cells, as well as for use in electrochromic devices, such as smart windows and flat panel displays. Additionally, the instant polymeric solid gel membranes are useful in rechargeable electrochemical cells, wherein the solid gel membrane is employed as a separator between the charging electrode and the anode.
With respect to a zinc/air fuel cell battery, for example, conductive membranes of the present invention may be used to protect the anode, as well as the cathode. In such a system, the ionic species is contained within the solution phase of the solid gel membrane, allowing it to behave as a liquid electrolyte without the disadvantages. The gel membrane protects the anode from corrosion (by the electrolyte as well as by air) and prevents zinc oxidation product from the anode from contaminating the electrolyte. With regard to the cathode, as the membrane is itself a solid, there is no water to block the air channels of the cathode. As a result, the system will have an extended lifetime.
As used herein, the term xe2x80x9canodexe2x80x9d refers to and is interchangeable with the term xe2x80x9cnegative electrodexe2x80x9d. Likewise, xe2x80x9ccathodexe2x80x9d refers to and is interchangeable with the term xe2x80x9cpositive electrodexe2x80x9d.
The present invention also includes rechargeable electrochemical cells that use the solid gel membrane as a separator between the anode and charging electrode. Such a separator provides many advantages that conventional separators lack. For example, it provides a smooth impenetrable surface that allows the exchange of ions for both discharging and charging of the cell while preventing fast dendrite penetration and the diffusion of reaction products such as metal oxide to remaining parts of the cell. Furthermore, the measured ionic conductivities of the present solid gel membranes are much higher than those of prior art solid electrolytes or electrolyte-polymer films. For example, the observed conductivity values for the present separators are surprisingly about 0.10 S/cm or more. Even more surprisingly, ionic conductivities as high as 0.36 S/cm have been measured, and it is possible that higher values still may be observed. Thus, these unique and unprecedented properties distinguish the separator of the present invention from previous designs that merely trap dendrite growth and slow penetration.
Accordingly, the principles of the present invention relate, in one aspect, to a rechargeable electrochemical cell comprising a separator, an anode, a cathode, and a charging electrode. Optionally, a liquid electrolyte, such as one of those mentioned herein and/or commonly known by those of skill in the art, may also be included in the rechargeable cell. The liquid (aqueous) electrolyte contacts the separator, each electrode, and a porous spacer, if employed. The separator comprises an ion-conducting polymer-based solid gel membrane which includes a support onto which a polymer-based gel having an ionic species contained within a solution phase thereof is formed. The support may be a woven or nonwoven fabric or one of the electrodes.
The polymer-based gel comprises a polymerization product of one or more monomers selected from the group of water soluble ethylenically unsaturated amides and acids. The polymer-based gel also includes a water soluble or water swellable polymer, which acts as a reinforcing element. In addition, a chemical polymerization initiator (listed below) may optionally be included. The ionic species is added to a solution containing the polymerization initiator (if used), the monomer(s), and the reinforcing element prior to polymerization, and it remains embedded in the polymer gel after the polymerization.
Polymerization is carried out at a temperature ranging from room temperature to about 130xc2x0 C., but preferably at an elevated temperature ranging from about 75xc2x0 to about 100xc2x0 C. Higher heating temperatures, such as those ranging from about 95xc2x0 to about 100xc2x0 C., provide a stiffer polymer surface, which is a desirable property in rechargeable cell applications. Optionally, the polymerization may be carried out using radiation in conjunction with heating. Alternatively, the polymerization may be performed using radiation alone without raising the temperature of the ingredients, depending on the strength of the radiation. Examples of radiation types useful in the polymerization reaction include, but are not limited to, ultraviolet light, xcex3-rays or x-rays.
In the rechargeable cell, the cathode and charging electrode may be a single bifunctional electrode or may be individual and separate electrodes. The separator is positioned between the anode and charging electrode. In alkaline systems, the hydroxide ionic species typically comes from an aqueous alkaline solution of potassium hydroxide, sodium hydroxide, lithium hydroxide, or combinations thereof. Preferably in a potassium hydroxide solution, for example, the base has a concentration ranging from about 0.1 wt. % to about 55 wt. %, and most preferably about 37.5 wt. %. In acidic systems, the proton comes from an aqueous acidic electrolyte solution, such as a solution of perchloric acid, sulfuric acid, hydrochloric acid, or combinations thereof. The concentration of perchloric acid, for example, preferably ranges from about 0.5 wt. % to about 70 wt. %, and most preferably about 13.4 wt. %. The membrane separator may also be used in neutral systems, wherein the ionic species comes from a saturated aqueous neutral solution of ammonium chloride and potassium sulfate; a saturated solution of ammonium chloride, potassium sulfate, and sodium chloride; or a saturated neutral solution of potassium sulfate and ammonium chloride.
When the cathode and charging electrode are individual and separate electrodes, the charging electrode is positioned between the separator and cathode, and a porous spacer is optionally positioned between the charging electrode and cathode.
In another aspect, the invention is a rechargeable electrochemical cell comprising a separator, a metal anode (preferably zinc), an air cathode, and a charging electrode. In this system, the separator is a hydroxide conducting polymer-based solid gel membrane comprising a support onto which a polymer-based gel having a hydroxide species contained within a solution phase thereof is formed. The polymer-based gel comprises polysulfone as a reinforcing element and a polymerization product of a polymerization initiator, methylenebisacrylamide, acrylamide, and methacrylic acid. The hydroxide species comes from an aqueous alkaline solution (ranging from about 0.1 wt. % to about 55 wt. % potassium hydroxide, sodium hydroxide, lithium hydroxide, or a mixture thereof), which is added to the polymerization initiator, methylenebisacrylamide, acrylamide, methacrylic acid, and polysulfone prior to polymerization. The air cathode and charging electrode may be a single bifunctional electrode or may be individual and separate electrodes. The separator is positioned between the metal anode and charging electrode. The ionic conductivity of the separator typically ranges from about 0.10 S/cm to about 0.36 S/cm, but may be higher.
In another aspect, the present invention is an electrochemical cell comprising first and second electrodes and one or more polymer based solid gel membranes disposed there between. In one embodiment, the electrochemical cell is a zinc/air cell having an anode protective solid gel membrane and a hydroxide conducting solid gel membrane disposed between the zinc anode and the air cathode. In another embodiment of a zinc/air system, both the anode and cathode are protected by a solid gel membrane of the present invention, and an aqueous electrolyte is disposed between the two.
In a further embodiment of this aspect of the invention, the electrochemical cell is an aluminum/air cell, wherein a hydroxide conductive solid gel membrane is applied to the aluminum anode to protect it from corrosion.
In yet a further embodiment of this aspect of the invention, the electrochemical cell is an aluminum/air cell, wherein a hydroxide conductive solid gel membrane is disposed between the aluminum anode and the air cathode.
Accordingly, the principles of the present invention also provide a method of inhibiting corrosion of a metal anode in a metal/air fuel cell system comprised of a metal anode and an air cathode. The method comprises disposing one or more polymer based solid gel membranes between said anode and said cathode.
In yet a further embodiment of the invention, the electrochemical cell is a proton or hydroxide conducting power source, such as a hydrogen fuel cell system. In this embodiment, a proton or hydroxide conductive solid gel membrane may be sandwiched between the hydrogen anode and the air cathode, thus separating the hydrogen and the air, while allowing the diffusion of proton or hydroxide ions. This embodiment provides several advantages over prior art proton conducting membranes in that the solid gel membranes of the present invention are much easier and less expensive to produce than earlier membranes and, more importantly, unlike previous membranes, the solid gel membranes of the present invention will function efficiently at room temperature.
The principles of the present invention may also be applied to electrochromic devices. Here, the electrochromic materials of the device are contained within solid gel membranes, thus providing the device with the reliability and long lifetime associated with solution phase EC systems, and also the energy-saving memory properties associated with thin-film EC systems.
Accordingly, yet another embodiment of the present invention is an electrochromic device wherein electrochromic materials are contained within polymer based solid gel membranes. Typically, such a device will involve two electrode substrates and electrochromic materials contained within solid gel membranes sandwiched there between. The device may optionally include an aqueous or a solid electrolyte disposed between the solid gel membranes. The electrode substrates may be comprised of such materials as, for example, platinum, gold, conductive glass, such as indium-tin oxide glass, and the like.